1. Field of the Invention
Embodiments of the invention relate to switching regulators and control devices of switching regulators.
2. Description of the Related Art
Known switching regulators to achieve high efficiency and low noise include a current resonant oscillation type that perform soft switching by using resonant oscillation operation with series-connected inductance and capacitance.
FIG. 12 shows an example of construction of a generally employed switching regulator of a current resonant oscillation type.
This regulator has a high side switch Qa and a low side switch Qb series-connected to form a switching circuit that is connected in parallel to a DC power supply Ed. The switches Qa and Qb have built-in body diodes Da and Db anti-parallel connected to the respective switches. A series-connected circuit composed of a primary winding P1 of a transformer T and a resonant oscillation capacitor Cr is connected in parallel to the low side switch Qb. A series connected resonant oscillation circuit is formed of a leakage inductance of the transformer T and the resonant oscillation capacitor Cr.
Secondary windings S1 and S2 of the transformer T are connected to a full-wave rectifying and smoothing circuit comprising diodes D1 and D2 and a smoothing capacitor Co. A load Ro is connected to the full-wave rectifying and smoothing circuit. The output side of the full-wave rectifying and smoothing circuit is also connected to a circuit for ON-OFF control of the switches Qa and Qb, the circuit comprising an error-amplifier circuit 1A, a voltage controlling oscillator circuit (VOC) 2A, a controller circuit 3A, and a driver circuit 4A.
This switching regulator sets a dead time in which both the switches Qa and Q b are in an OFF state, and the switches repeat ON and OFF operation alternately with a duty factor of about 50%. Current resonant oscillation operation takes place in the circuit including the resonance capacitor Cr and the leakage inductance of the primary winding P1 and the secondary windings S1 and S2 of the transformer T, performing power transmission from the primary side to the secondary side of the transformer T. The output from the secondary side of the transformer T is rectified by the diodes D1 and D2 and made smooth by the smoothing capacitor Co to deliver DC output voltage. This output voltage is detected by the error amplifier circuit 1A. The voltage controlling oscillator circuit 2A controls the oscillation frequency thereof corresponding to this output voltage. The controller circuit 3A and the driver circuit 4A perform alternating ON-OFF control of the two switches Qa and Q b. Thus, the output voltage is stabilized.
One of the advantages of the switching regulator of the current resonant oscillation type is that the regulator performs soft switching by using the body diodes Da and Db of the switches Qa and Qb. In the state the high side switch Qa is in an OFF state and the low side switch Qb is in an ON state with the current I_Qb flowing through the switch Qb in the direction of the arrow in FIG. 12, if the low side switch Qb is turned OFF from this state, then the current I_Qb that has been flowing in the switch Qb turns to flow through the body diode Da of the high side switch Qa. Since the voltage Vs between the switch Qa and the switch Qb is approximately the voltage Vi of the DC power supply Ed during a current is flowing in the body diode Da, even if the switch Qa is turned ON, the voltage across the switch Qa does not change rapidly. Thus zero voltage switching (ZVS) is achieved. Similarly, when the current I_Qa that has been flowing in the high side switch Qa is transferred to the body diode Db of the low side switch Qb, the voltage Vs between the switch Qa and the switch Qb becomes approximately the ground potential. Consequently, if the switch Qb is turned ON during a current is flowing in the body diode Db, the voltage across the switch Qb does not change rapidly, achieving zero voltage switching.
However, if the switch Qa or the switch Qb turns ON at a moment when the voltage Vs between the switch Qa and the switch Qb is a certain voltage between the voltage Vi of the power supply Ed and the ground potential, the switching is a hard switching. In this condition, rapid change arises in the current through the switch Qa or the switch Qb and in the voltage across the switch Qa or the switch Qb. This results in generation of noise and increase in the power loss at the switch Qa or the switch Qb. In addition, if the low side switch Qb turns ON during a current is running through the body diode Da of the switch Qa, a short-circuit current flows through the body diode Da from the DC power supply Ed to the switch Qb in the recovery time of the body diode Da. This short-circuit current can be an instantaneous heavy current causing destruction of the switches Qa and Qb.
Some proposals have been made to cope with the hard switching and the short-circuit current. Japanese Unexamined Patent Application Publication No. 2005-051918, also described herein as Patent Document 1, discloses a power supply in which a state of current flow through a body diode is detected by detecting a current through a resonant oscillation circuit and, during detecting the state, no driving signal to turn either of the two switches ON or OFF is generated. Japanese Unexamined Patent Application Publication No. 2010-004596, also referred to herein as Patent Document 2, discloses a power supply that does not generate short-circuit current. In this power supply, the ON-width of a turning ON signal for a low side switch is controlled relatively long immediately after start-up to inhibit turning ON of a high side switch during a period in which the current flowing through the low side switch is negative. Japanese Unexamined Patent Application Publication No. 2009-027803, also referred to herein as Patent Document 3, discloses a power supply in which solely a high side switch is turned ON and OFF immediately after start-up. After a resonant oscillation capacitor is changed to a level that allows normal resonant oscillation operation, switching operation using two switches is conducted. Japanese Translation of PCT Application Publication No. 2007-527190, also referred to herein as Patent Document 4, discloses a device that can deal with both hard switching and short-circuit current by directly detecting the voltage at a point between two switches. Japanese Unexamined Patent Application Publication No. 2007-006614, also referred to herein as Patent Document 5, discloses a power supply to cope with short-circuit current by detecting the voltage at a point between two switches.
However, the construction disclosed in Patent Document 1, which involves a resistor for current detection in the resonant oscillation circuit, causes a problem of power loss due to the resistor. The construction disclosed in Patent Document 2, in which the ON-width of a turn ON signal is adjusted immediately after start-up, only deals with the operation at the start-up, thus application period is limited. The construction disclosed in Patent Document 3, which performs resonant oscillation operation after charging the resonant oscillation capacitor, also limits application period to the time immediately after start-up. The constructions disclosed in Patent Document 4 and Patent Document 5, which involve detecting high voltage between switches, need to have a control circuit including high voltage circuit elements. This causes an enlarged scale of the control circuit and adversely affects terminal construction of an integrated circuit for the control circuit.